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ISSN 1744-683X
www.rsc.org/softmatter Volume 8 | Number 31 | 21 August 2012 | Pages 7991–8242
Volume 8 | Number 31 | 2012 Soft Matter Themed issue: Hydrogel mechanics Pages 7991–8242
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Highlighting joint research results from the
Department of Chemistry and Biotechnology,
Yokohama National University, Japan, and the
Department of Chemistry and Department of
Chemical Engineering and Materials science,
University of Minnesota, Minnesota.
Title: Thermoreversible high-temperature gelation of an
ionic liquid with poly(benzyl methacrylate-b-methyl
methacrylate-b-benzyl methacrylate) triblock copolymer
A novel thermosensitive triblock copolymer and ionic liquid
composite exhibits a reversible low-temperature-sol–high-
temperature-gel transition.
As featured in:
See Masayoshi Watanabe et al.,
Soft Matter, 2012, 8, 8067.
Themed issue: Hydrogel mechanics
1744-683X(2012)8:31;1-A
PAPER
Jian Ping Gong et al.
Swelling-induced long-range ordered structure formation in
polyelectrolyte hydrogel
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Swelling-induced long-range ordered structure formation in polyelectrolyte
hydrogel†
Md. Arifuzzaman,
a
Zi Liang Wu,
a
Takayuki Kurokawa,
bc
Akira Kakugo
bd
and Jian Ping Gong
*
bc
Received 6th April 2012, Accepted 10th May 2012
DOI: 10.1039/c2sm25814e
A millimeter-scale periodic structure is created in a polyelectrolyte hydrogel by the rapid-heterogeneous
swelling process, and is frozen by the polyion complexation of the polyelectrolyte network with the
oppositely charged, semi-rigid polyelectrolyte. The hydrogel is synthesized from a cationic monomer,
N-[3-(N,N-dimethylamino)propyl] acrylamide methyl chloride quaternary (DMAPAA-Q), in the
presence of a small amount of the oppositely charged poly(2,2
0
-disulfonyl-4,4
0
-benzidine
terephthalamide) (PBDT) that has a semi-rigid nature. During the swelling process, surface creasing due
to the large mismatching of swelling degree between the surface layer and the inner one of the poly
DMAPAA-Q (PDMAPAA-Q) gel occurs, which induces highly oriented semi-rigid PBDT molecules
along the tensile direction of the crease pattern. To accompany the evolution of surface creasing,
a lattice-like periodic birefringence pattern is formed, which is frozen permanently by the strong
polyion complex formation, even after the surface instability pattern of the gel disappears completely
throughout the dynamic coalescence. In this work we rationally clarified that formation of such a long-
range ordered non-equilibrium structure in the polyelectrolyte hydrogel by the rapid-heterogeneous
swelling process requires the following three indispensable conditions: (i) swelling-induced surface
creasing; (ii) polyion complex formation; and (iii) a semi-rigid or rigid dopant. This sort of non-
equilibrium structure formation mechanism may help understand how biomacromolecules that are
rigid polyelectrolytes, such as deoxyribonucleic acid, microtubules and actin filaments, form rich
architectures during the growth of biological organs.
Introduction
Biological soft tissues contain a certain amount of water that
ensures molecular mobility and at the same time, possess
a sophisticated structure that enables them to exhibit
outstanding performance over a wide range of physiological
functions.
1–4
Among all, intricate structural patterns found in the
human brain, the intestine, and leaves, are believed to be formed
via a non-equilibrium, dynamic process during growth,
5,6
and are
frozen by the physical/chemical interaction among the mole-
cules.
7,8
Elucidation of the mechanism of such structure
formation induced by non-equilibrium chemistry in living bodies
is an attractive research topic. Furthermore, introducing
sophisticated structures into hydrogels,
9–12
which are soft, wet
materials similar to the biological tissues, by the non-equilibrium
process is one of the ultimate challenges for polymer scientists.
In our previous study, we found that a piece of sheet-like
polyelectrolyte gel containing an oppositely charged semi-rigid
polymer exhibits a periodic birefringence pattern after full
swelling in water.
13
Such a well-ordered structure with milli-
meter-scale periodicity was initially formed in a polyelectrolyte
gel with a dilute concentration (water content >90 wt%).
Although some plausible explanation for the structure formation
could be derived, the exact structure formation mechanism
remains a mystery.
In this study, we showed that the periodic birefringence
pattern, representing an ordered orientation of the semi-rigid
poly(2,2
0
-disulfonyl-4,4
0
-benzidine terephthalamide) (PBDT)
molecules, developed during swelling of the hydrogel. Further,
we found that the surface mechanical instability induced by the
rapid-heterogeneous swelling, the semi-rigid nature of the dopant
macromolecule, and polyion complex formation are the three
indispensable requirements for the formation of such an ordered
structure. This study reveals a novel strategy for introducing
a
Laboratory of Soft and Wet Matter, Division of Biological Sciences,
Graduate School of Science, Hokkaido University, Sapporo 060-0810,
Japan
b
Laboratory of Soft and Wet Matter, Faculty of Advanced Life Science,
Graduate School of Science, Hokkaido University, Sapporo 060-0810,
Japan. E-mail: gong@mail.sci.hokudai.ac.jp
c
Creative Research Institution (CRIS), Hokkaido University, Sapporo
001-0021, Japan
d
PRESTO, Japan Science and Technology Agency (JST), Japan
† Electronic supplementary information (ESI) available: Brief discussion
on (i) the POM observations of the as-prepared state, (ii) experimental
results by the controlled (slow) swelling kinetics, and (iii) movie files
recorded during rapid swelling are presented in the ESI. See DOI:
10.1039/c2sm25814e
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a long-range ordered structure in amorphous hydrogels through
dynamic mechano-complexation coupling in a non-equilibrium
process. This strategy can be applied to soft, wet systems in
a variety of contexts, namely, for developing various ordered
structures with macroscale correlation by proper designing of the
hydrogel geometry. Furthermore, rigid or semi-rigid macro-
molecules such as deoxyribonucleic acid (DNA), microtubules
(MTs), and actin filaments (F-actin) are ubiquitous in living
organisms. These biomacromolecules form rich architectures
through self-assembly or via physical/chemical interactions with
other species. The mechanism of non-equilibrium structure
formation elucidated in this study might provide scientific
insights into the formation of rich architectures by these bio-
macromolecules during the growth of biological organs.
Experimental
Materials
PBDT, a water-soluble semi-rigid polyanion, was synthesized by
an interfacial polycondensation reaction.
14
The synthesized
PBDT had an average molecular weight, M
w
, of about 1.6
10
6
g mol
1
and a liquid crystalline critical concentration C
LC
*of
2.8 wt% in aqueous solution.
13,15,16
The cationic monomer,
N-[3-(N,N-dimethylamino)propyl]acrylamide methyl chloride
quaternary (DMAPAA-Q) (Kohjin Co. Ltd. Japan), the anionic
monomer, 2-acrylamido-2-methylpropanesulfonic acid (AMPS)
(Tokyo Kasei Co. Ltd.), the neutral monomer, acrylamide
(AAm) (Junsei Chemical Co. Ltd. Japan), and the photoinitiator,
2-oxoglutaric acid (OA) (Wako Pure Chemical Industries Ltd.
Japan), were used as received without further purification. N,N
0
-
Methylenebis(acrylamide) (MBAA) (Wako Pure Chemical
Industries Ltd. Japan) was recrystallized from ethanol and used
as a chemical cross-linker. For all the experiments, deionized
water, purified using 0.22 mm and 5 mm membrane filters, was
used.
Synthesis of the sheet-like polyelectrolyte gels
To synthesize the poly DMAPAA-Q (PDMAPAA-Q) gel con-
taining PBDT, an aqueous solution was prepared by mixing the
cationic monomer, DMAPAA-Q (2.0 M), the semi-rigid anionic
polymer, PBDT (1.0 wt%), the chemical cross-linker, MBAA
(2.0 mol%), and the photoinitiator, OA (0.15 mol%) together (the
amount in mol% is related to the cationic monomer concentra-
tion). After proper mixing, the precursor solution was poured
into a reaction cell consisting of a 1.0 mm thick rectangular
silicone rubber frame sandwiched between a pair of parallel glass
plates. Before polymerization, the precursor solution was trans-
parent and completely amorphous because the PBDT concen-
tration was much lower than the liquid crystalline critical
concentration (C
LC
* ¼ 2.8 wt%). In the precursor solution the
charge ratio of PBDT to DMAPAA-Q was 0.02, which was far
below the stoichiometric value. Radical polymerization was
effected by UV irradiation from both sides of the glass reaction
cell for 6.0 h at room temperature in argon atmosphere. Other
gels with sheet-like shapes [poly AMPS (PAMPS) and poly AAm
(PAAm) gels in the presence of the semi-rigid anionic polymer
PBDT and PDMAPAA-Q gel in the presence of the flexible
anionic polymer PAMPS] were synthesized by using the same
method and conditions.
Swelling of the polyelectrolyte gels
After prolonged UV polymerization, the sheet-like gels (about
45 45 1.0 mm
3
) were carefully removed from the glass
reaction cell and cut into specific dimensions (about 6.0 3.0
1.0 mm
3
) using a mechanical gel cutter (Dumb Bell Co. Ltd.
Japan). Then, the gels were immediately immersed in water for
spontaneous swelling. The volume of water (200 mL) was kept
constant for every sample. In the case of slower kinetics, to
decrease the rate of swelling, a confined metal chamber in which
equilibrated humid conditions were successfully maintained for
a long time was used.
Measurement of swelling kinetics
At room temperature, a set of as-prepared gels with dimensions
of about 6.0 3.0 1.0 mm
3
were immersed together in a large
quantity of water for swelling. The increased mass of the samples
at different swelling times (t) was recorded by measuring the
weight of the samples on an electronic balance (Shimadzu Co.,
Kyoto, Japan). The relative swelling ratio, q (g/g), is defined as
the ratio of the weight of the gel swollen for different time lengths
to its weight in the as-prepared state.
17
Polarizing optical microscope (POM) observation
The surface morphology and the birefringence of the gels in the
as-prepared state, during swelling, and finally in the equilibrium
swelling state were observed under a polarizing optical micro-
scope (POM) (Nikon, Eclipse, LV100POL) in the parallel and
crossed polarization modes, respectively, at room temperature.
To determine the orientation direction of the rod-like PBDT
molecules or their fibrous bundles, the samples were also
observed under a crossed POM with a 530 nm sensitive tint plate.
The change in the surface morphology of the gels during unre-
stricted and rapid swelling was observed under the POM with
a parallel polarizer at room temperature. To observe such
a swelling-induced surface instability crease pattern, the surface
of the as-prepared gels was fully covered with an aqueous solu-
tion of 0.05 wt% Alcian blue (Wako Pure Chemical Industries
Ltd. Japan). Therefore, the crease pattern with its sharp
boundary was clearly observed in the gel during swelling. Images
were captured by a digital camera coupled with the microscope.
Results and discussion
To elucidate the correlation between the surface creasing and the
semi-rigid molecular orientation at different swelling times in
water, we observed the sample under a polarizing optical
microscope (POM) in three different modes: a parallel polari-
zation mode to identify the surface morphology [Fig. 1, column
(i)], a crossed polarization mode to identify the birefringence
[Fig. 1, column (ii)], and a crossed polarization mode with
a 530 nm sensitive tint plate to identify the orientation of the
semi-rigid PBDT molecules [Fig. 1, column (iii)]. The schematic
orientation of the PBDT molecules is shown in Fig. 1,
column (iv).
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The as-prepared PBDT-containing PDMAPAA-Q gel was
always transparent with a smooth surface [Fig. 1a(i)] and
exhibited irregular birefringence [Fig. 1a(ii and iii)] that was
almost unchanged with the rotation of the sample on the POM
stage (see ESI, Fig. S1†).
When a sample of the as-prepared gel, 6.0 3.0 1.0 mm
3
in
size, was immersed in water, rapid swelling of the gel commenced
immediately and polygonal patterns appeared on the gel surface,
with clear lines that corresponded to the borders of the crease
patterns [Fig. 1b(i)]. Simultaneously with the appearance of the
polygonal patterns, the entire irregular birefringence pattern
rapidly changed into numerous distinct small domains (about
200–300 mm in length) having leaf-like shapes. These leaf-like
patterns always appeared around the borderlines of the crease
patterns [Fig. 1b(ii) and see ESI, Movie S1†]. The birefringence
of the PDMAPAA-Q gel containing PBDT is related to the
orientation of the semi-rigid PBDT molecules, which is typical of
that of a positive liquid crystal.
15,16,18–21
From the characteristic
birefringence colors in the presence of the 530 nm sensitive tint
plate [Fig. 1b(iii)], we concluded that the PBDT molecules are
aligned perpendicular to the borderlines, as illustrated in
Fig. 1b(iv).
As the swelling time increased, these polygonal surface
patterns coalesced into larger ones ca. 1.0 mm in length
[Fig. 1c(i)], exhibiting even more clear, lattice-like borders cor-
responding to the cubic packing of the convex creases. Consistent
with this surface pattern evolution, the leaf-like domains in the
birefringence image coalesced into larger ellipsoidal domains,
Fig. 1 Time evolution of surface morphology and ordered structure formation for PBDT-containing PDMAPAA-Q hydrogel during swelling in water.
The observation was performed under the polarizing optical microscope (POM). The images presented in column (i), observed under the parallel
polarizer, show the surface morphology of the gel; column (ii), observed under the crossed polarizer, shows the birefringence of the gel; column (iii),
observed under the crossed polarizer with a 530 nm sensitive tint plate, shows the molecular orientation direction. Column (iv) is a schematic illustration
of the orientation of semi-rigid PBDT molecules, where the green single bars represent the weak molecular orientation of PBDT, while the blue and
orange bars indicate strong/highly ordered orientation. To visualize the surface morphology, the as-prepared gel was swollen in water containing a dye,
Alcian blue. The dimensions of the as-prepared gels were about 6.0 mm (L) 3.0 mm (W) 1.0 mm (T). A: analyzer, P: polarizer, X
0
and Z
0
: fast and
slow axes of the tint plate, respectively. All the images are shown on the same scale as in a(i).
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with the borderline as the long axis of the ellipsoid and the PBDT
molecules oriented vertically to this axis [Fig. 1c(iii)]. After
prolonged swelling, the surface crease pattern disappeared along
with the lattice-like borders, and the gel surface became smooth
and flat [Fig. 1d(i)], while the birefringence pattern remained
even after reaching the equilibrium swelling state, keeping with
the high periodicity [Fig. 1d(ii and iii)].
To further confirm the lattice-like symmetry of the oriented
structure, the angle dependence of the finally stable birefringence
pattern was investigated. With every 90
rotation, the same
periodic birefringence pattern appeared, while the pattern
became less regular with a 45
rotation in the clockwise or
anticlockwise direction (see ESI, Fig. S2†). These observations
confirmed that the gel has a periodically ordered structure with
a square-shaped lattice unit, whereupon the PBDT molecules
orient vertically with respect to the lattice boundary. One of the
periodical units in the equilibrium state is highlighted by the
dashed boxes in Fig. 1d(ii and iv). Thus, our previously proposed
structure needs modification.
13
Next, we carried out a more quantitative investigation of the
correlation between the swelling kinetics and the ordered struc-
ture formation in the gel. For a diffusion-limited process, the
characteristic swelling time is determined by the smallest
dimension of the sample, which in the present case is the sample
thickness. The time profile of the swelling ratio q in terms of the
weight change of the gel relative to the as-prepared state is shown
in Fig. 2(a). The gel swelled rapidly and reached its equilibrium
swelling state within 20 min [Fig. 2(a), inset]. Evolution of the
surface morphology and the birefringence pattern after the initial
600 s are shown in Fig. 2(b) and (c), respectively. As shown in
Fig. 2(b), bumped irregular polygons appeared on the surface
during the initial period of the swelling process (30 s). With the
advancement of swelling (200 s), the irregular polygons fused
rapidly to form regular patterns with lattice borderlines. In order
to characterize the coalescence of the surface crease patterns, we
define n
JP
as the density of junction point (the number of
crossover junction points of the creases per square millimeter
area), where at least three bumps coexist. The value of n
JP
decreased very rapidly with time and finally reduced to zero after
a swelling time of about 400 s [Fig. 2(b)].
The leaf-like birefringence pattern appeared at around 60 s
and became regular at around 200 s [Fig. 2(c)] like a symmetric
lattice. The leaf length or the length of the lattice unit L
s
increased gradually with the swelling time [see ESI, Movie S2†],
reaching a constant value of 1.1 mm at around 400 s [Fig. 2(c)],
which was approximately the same time taken for the disap-
pearance of the surface crease pattern.
From the above observations, we conclude that a periodic
birefringence pattern is formed during the swelling process and
that it is correlated to surface creasing. The latter is often tran-
sient in nature during the swelling of hydrogels, especially in the
case of polyelectrolyte hydrogels that have a very high osmotic
swelling pressure.
22–39
The rapid swelling of the surface layer
causes a large difference in the degree of swelling between the
surface and the inner layer of the gel; this spatial mismatch results
in compressive strain in the surface layer and leads to transient
creasing instability in the surface layer.
22–39
Theoretical and
experimental studies indicate that the onset of creasing is only
related to the effective compressive strain experienced by the
surface layer, irrespective of the modulus and thickness of the
mismatched layers, while the characteristic spacing between the
creases increases with the surface layer thickness. The critical
value of compressive strain for the creasing instability, 3
c
,is
reported as 0.35.
22,24,29,35–39
At the initial stage of swelling, the
high osmotic swelling pressure induces rapid swelling to satisfy
3 > 3
c
; thus, creasing instability occurs and polygonal patterns
appear. With the advancement of swelling, the thickness of the
surface swollen layer increases, and this leads to coalescence and
eventual disappearance of the crease patterns at equilibrium
Fig. 2 Time profiles of swelling ratio (q) relative to the as-prepared state
(a), density of junction point (n
JP
) of the crease patterns on the surface
(b), and the length of the lattice unit (L
s
) of the birefringence images (c) of
the PBDT-containing PDMAPAA-Q gel during swelling in water. Inset
graph in (a) represents the identical swelling process of the gel for
a prolonged period. Images in (b) and (c) were observed by a POM with
a parallel and crossed polarizer, respectively. The scale bar in the inset
images represents 500 mm, and all the images are shown on the same scale.
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swelling because of the disappearance of the swelling mismatch.
It is interesting that in the present case, before disappearing, the
polygonal patterns coalesce to form cubic patterns instead of
hexagonal patterns, which is the previously reported mode at the
instability onset.
22,23
We also found that the size of the millimeter-scale periodicity
increased linearly with the sample thickness but did not change
with the formulation of the gel, such as the concentrations of the
cationic monomer and the semi-rigid anionic polymer in the
precursor solution.
13
These results are in agreement with the
feature of the creasing instability.
35–39
To confirm that the surface creasing originated only from the
rapid swelling of the polyelectrolytic gel PDMAPAA-Q and that
the semi-rigid PBDT did not play a role in the same, we studied
the swelling behavior of the PDMAPAA-Q hydrogel without
PBDT [Fig. 3(a)]. No birefringence was observed in the as-pre-
pared state [Fig. 3a(i)], indicating the amorphous nature of the
PDMAPAA-Q hydrogel. A transient surface crease pattern and
birefringence pattern similar to those for PDMAPAA-Q con-
taining PBDT [Fig. 1c(ii)] were observed at the initial stage of
swelling, although the birefringence pattern was relatively weak
[Fig. 3a(ii)]. However, both the patterns disappeared at equilib-
rium swelling [Fig. 3a(iii)].This result indicated that the periodic
birefringence pattern is induced by the creasing instability of the
PDMAPAA-Q gel, while the strong polyion complexation
between the positively charged PDMAPAA-Q network and the
Fig. 3 Effect of surface instability, polyion complexation, and rigidity of the dopant molecule on the regular-stable birefringence pattern formation in
the hydrogel during the swelling process. The positively charged PDMAPAA-Q gel alone [a(i)] or in the presence of the negatively charged flexible
dopant PAMPS [b(i)] exhibits almost no birefringence in the as-prepared state. Upon swelling, a transient birefringence pattern appears in the gels [a(ii)
and b(ii)] because of the surface creasing instability. However, both the gels become amorphous well ahead of equilibrium swelling [a(iii) and b(iii)]. In
presence of the negatively charged semi-rigid PBDT, the as-prepared anionic PAMPS and neutral PAAm gels exhibit weak birefringence patterns [c(i)
and d(i), respectively]. During swelling, the birefringence pattern in the PBDT-containing PAMPS gel changes into numerous leaf-like patterns [c(ii)]
because of the strong surface creasing instability [same as the PBDT-containing PDMAPAA-Q gel in Fig. 1c(ii)]. In contrast, the birefringence pattern
remains almost unchanged in the PBDT-containing PAAm gel [d(ii)] since there is no significant surface instability. However, after equilibrium swelling,
both the PAMPS and PAAm gels containing PBDT show completely amorphous characteristics [c(iii) and d(iii), respectively] owing to the absence of
polyion complexation. In all the cases, the dimensions of the as-prepared gels are the same as those in Fig. 1. A: analyzer, P: polarizer. All the images are
shown on the same scale as in a(iii).
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negatively charged semi-rigid molecule PBDT freezes the bire-
fringence pattern. Certainly, polyion complexation is enhanced
by the outward diffusion of the mobile counterions (Na
+
and
Cl
) from the gel to the solvent environment during the swelling
process,
40–42
which stabilizes the molecular orientation inside the
gel. Thus, the intense polyion complexation between PDMA-
PAA-Q and PBDT freezes the periodic molecular orientation
induced by the mechanical instability.
Furthermore, we have clarified that the semi-rigid nature of
PBDT plays an indispensable role in the stabilization of the
birefringence pattern. As shown in Fig. 3b, the PDMAPAA-Q
hydrogel containing a negatively charged flexible linear polymer,
polyAMPS (PAMPS), exhibited no birefringence in the as-
prepared state [Fig. 3b(i)], while the crease pattern and bire-
fringence pattern appeared transiently during swelling
[Fig. 3b(ii)]. However, both patterns disappeared well ahead of
the equilibrium swelling state [Fig. 3b(iii)]. This result suggests
that the semi-rigid nature of the PBDT plays an important role in
maintaining the highly oriented structure in the gel. As has been
elucidated, mesoscopic fibrous bundles formed from semi-rigid
PBDTs are very sensitive to external stress–strain and show
a stronger birefringence owing to specific molecular orientation
than do the flexible molecules.
43
The strong birefringence
observed in Fig. 1 indicates that the semi-rigid PBDT forms long
and rigid fibrous bundles in the tensile direction of creasing,
almost vertical to the borders of the crease patterns.
We also confirmed the effect of polyion complexation by
investigating the like-charged combination. The gel synthesized
from an anionic monomer, AMPS in the presence of the semi-
rigid PBDT also exhibited certain birefringence in the as-
prepared state [Fig. 3c(i)] and an creasing instability pattern
when exposed to water [Fig. 3c(ii)]. However, after sufficient
(equilibrium) swelling, both the surface instability and birefrin-
gence pattern disappeared [Fig. 3c(iii)]. Such an amorphous
phenomenon can be explained by the absence of polyion
complexation between the semi-rigid anionic PBDT and the
chemically cross-linked negatively charged PAMPS. We also
confirmed that in the absence of creasing instability, for example,
in the case of the PDMAPAA-Q gel swollen with water vapor
(see ESI, Fig. S3†) or the neutral PAAm gel, no periodic bire-
fringence pattern appeared, even though these gels contained
PBDT [Fig. 3d]. Table 1 summarizes the results obtained for
various systems.
Conclusion
In conclusion, a long-range periodic structure is induced during
the swelling of the polyelectrolyte gel, concomitantly with the
occurrence of surface creasing, a transient process related to the
mechanical instability during the free and rapid swelling of
polyelectrolytic gels in water. The mismatch in the swelling ratio
between the surface layer and the inner layer of the gel induces
creasing instability, which exerts tension on the semi-rigid
molecules vertical to the borderline of the crease patterns. Thus,
the molecules orient along the tensile direction and exhibit strong
Table 1 Surface morphology and birefringence pattern of various hydrogels with different dopant molecules. A swelling-induced permanent and highly
ordered macroscopic structure is obtained only for the PDMAPAA-Q gel containing PBDT. Here, the circles and crosses indicate ‘‘yes’’ and ‘‘no,’’
respectively. The symbols (+), (), and (0) indicate positively charged, negatively charged, and neutral polymers, respectively
Hydrogel
Polyelectrolyte
dopant Crease pattern
Lattice-like birefringence
During swelling At equilibrium
PDMAPAA-Q (+) PBDT() BB B
PBDT()
a
PAMPS() BB
None BB
PAMPS() PBDT() BB
PAAm(0) PBDT()
a
Slow swelling in water vapor.
Fig. 4 Mechanism of the formation of swelling-induced ordered struc-
ture in a polyelectrolyte gel containing a very small amount of an oppo-
sitely charged semi-rigid polyelectrolyte. When the sheet-like gel is
immersed in water, rapid swelling occurs in the surface layer owing to the
high osmotic pressure of the polyelectrolytic gel. The rapid swelling of the
surface layer induces a large mismatch in the internal stress between the
surface layer and the inner layer. Therefore, surface creasing instability
occurs, which exerts tensile stress vertical to the boundary of the crease
patterns and induces polymer network orientation. In the presence of an
oppositely charged semi-rigid polyelectrolyte as the dopant, this polymer
network orientation is frozen by the subsequent polyion complexation
and fibrous bundle formation between the two oppositely charged
components, which is stabilized by the removal of the low-molecular
counterions through diffusion from the gel. Thus, the ordered structure
remains even after the surface crease pattern completely disappears in the
equilibrium swelling state. The dotted red arrows indicate the direction of
tension induced by the formation of the crease pattern.
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periodic birefringence. The molecular orientation is fixed
simultaneously by the strong ion complexation between the two
oppositely charged components and the formation of a lateral
fibrous bundle from these components; the semi-rigid nature of
the dopant molecules is indispensable for the fibrous bundle
formation. Thus, the strong birefringence pattern is frozen even
after the disappearance of the surface creasing instability. Fig. 4
illustrates the mechanism of formation of the swelling-induced
ordered structure in the polyelectrolyte gel.
These results might help derive a novel strategy for introducing
an ordered structure in soft, wet systems using rigid macromol-
ecules, coupling with mechanical instability and ion
complexation.
Acknowledgements
This research was supported by a Grant-in-Aid for the Specially
Promoted Research (no. 18002002) from the Ministry of
Education, Science, Sports and Culture of Japan. The authors
are thankful to Mr Yukihiro Nakano for informative and valu-
able discussions.
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8066 | Soft Matter, 2012, 8, 8060–8066 This journal is ª The Royal Society of Chemistry 2012
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